The invention relates to monitoring properties of insulating films including oxide layers of the type deposited or otherwise formed on a semiconductor substrate. In particular, the invention relates to monitoring those properties by accurately measuring the surface potential of the insulating film.
The majority of semiconductor devices including MOS (metal oxide semiconductor) capacitors, transistors, and corresponding integrated circuits incorporate insulating films (e.g., SiO.sub.2) deposited or grown on semiconductor substrate wafers (e.g., silicon). Device manufacturing processes require precise monitoring of film properties, such as film capacitance, electric charges in the film, as well as, the charge at the semiconductor substrate film interface. Non-contact measurements of these properties are performed by placing an electrostatic charge on the film surface with a corona discharge in the air followed by measuring the corresponding change in the surface potential with a vibrating Kelvin or Monroe type electrode positioned near the surface of the film. During the measurement, the wafer is held, typically by vacuum suction, on an electrically conducting wafer chuck connected to the ground potential. For precise measurements, the semiconductor substrate potential is required to be the same as the ground potential. This is often difficult to achieve if a thick insulating film is present on the wafer back-surface contacting the chuck. Such a configuration with a wafer sandwiched between two isolating films is commonly referred to as the so-called "floating wafer arrangement." Electrostatic potential of such a wafer is susceptible to undesirable changes caused by various static related events interfering with the measurements. To improve the accuracy of these measurements, an electrically conductive probe can be used to penetrate very thin films (e.g., less than 200 .ANG.) in order to make contact with the semiconductor substrate and equalize the wafer potential to the ground potential.